Heat exchangers and other fluid-handling equipment are often fitted with tube bundles that are fabricated by the attachment of a collection of tubes to one or two tubesheets. The tubes in the bundle may be arranged in two primary layouts: rectangular (also referred to as inline arrangement) and triangular (also referred to as staggered arrangement). The rectangular arrangement leads to tubes that are aligned vertically and horizontally while the triangular arrangement sets three adjacent tubes at the vertices of an equilateral triangle.
The tube bundle may be fitted with a series of baffles that provides support to the tubes while also diverting the shellside flow across the tube bundle in a serpentine manner. The firm attachment of tubes to the tubesheets enables the use of two separate fluids: one on the shellside and the second on the tubeside. Such an exchanger, therefore, allows energy transfer across the tube wall thickness from one fluid to the other.
The shellside fluid, for the most part, flows orthogonal to the tubes in the bundle. Such a crosswise flow tends to create unstable eddies on the downstream side of some or all tubes. Under some conditions, such eddies may lead to vibration of the tubes. Depending on the intensity of these vibrations, some tubes may experience excessive displacements from their original (i.e., with no flow condition) locations and eventually lead to tube failures.
Tube damage owing to flow-induced vibration can occur very quickly (several days or weeks) or take several years. Also, a properly designed exchanger may experience such failures owing to a recent increase in shellside flow rate, the flow conditions, tube wall corrosion, etc. Tube failures often require isolation of the exchanger for repair thereby causing costly downtime (e.g., production losses) and the associated repair costs as well as operational risks.
In their U.S. Pat. Nos. 7,032,655, 7,128,130, and 7,267,164, Wanni, et al. provide background on the state-of-the-art tube support devices available on the marketplace for vibration mitigation in tube bundles. Those patents are incorporated by reference here for their disclosures where necessary to provide background to the present invention.
Such prior art also discuss in detail the weaknesses of those technologies available in the marketplace. While the technologies presented by Wanni, et al. solve most of the problems they had described, the new technologies proposed by Wanni, et al. also have several limitations.
First, the dimples located on one end of the tube support as well as the protrusions along the remaining length require substantial amount of metal deforming (starting from a flat thin strip of metal), especially, for the rectangular tube arrangement which has the widest tube lanes. As a result, proper forming of dimples would be difficult for harder metallurgies, such as Titanium and Duplex Steel, where penetrating cracks are difficult to avoid.
Second, fabrication of tube support requires a very high pressing force (in excess of 200 tons representing expensive machinery), especially, when a single pressing operation is used. The use of multiple pressings along the length of the tube support decreases the required pressing force, but this lowers the technical quality of the stake and also increases the production time substantially.
Third, fabrication of these tube supports requires the use of high-precision die sets that are very expensive and they will wear out and can be damaged with repeated use.
Fourth, these tube supports cannot properly support those tubes that are located very close to the shell inner diameter when U-tube bundles are involved; this is because some of the tubes can be located only ⅛″ (3.2 mm) away from the shell inner diameter, but the dimples have a thickness of ⅜″ (9.5 mm) in this region.
Fifth, the dimples at the outer end of the tube support do not provide an adequately robust locking with the nearby tubes so that the stake can pivot around this outer end and rotate under some conditions, especially when the tube support has a somewhat loose fit within the tube bundle.
Sixth, the equipment used for the purpose of fabricating the tube supports with the dimples and protrusions does not lend itself easily to achieve an adjustable overall thickness for the tube support.
Seventh, once the tube support is formed, its overall thickness, that determines the extent of vibration mitigation, cannot be adjusted in any way. This could lead to inadequate tube support in certain regions of a tube bundle or additional tube support must be used to achieve an acceptable level of vibration mitigation.
In their U.S. Pat. Nos. 7,506,684 and 7,793,708, Wanni, et al. provide an alternate stake design that is limited only to tube bundles having the rectangular tube arrangement.